VLBA > Baseline Sensitivity

# Baseline Sensitivity

$$\Delta S = {\rm SEFD} / [\eta_s \cdot ( 2 \cdot \Delta \nu \cdot \tau_{\rm ff} ) ^{1/2} ] \; {\rm Jy}$$
SEFD or "system equivalent flux density" is the system noise expressed in Janskys.  $\eta_s \le 1 \ \$ accounts for the VLBI system inefficiency (primarily quantization in the data recording).  Kogan (1995b) provides the combination of scaling factors and inefficiencies appropriate for VLBA visibility data.  The bandwidth in Hz is $\Delta\nu$.  For a continuum target, use the channel bandwidth or the full recorded bandwidth, depending on the fringe-fitting mode; for a line target, use the channel bandwidth divided by the number of spectral points that span the channel.  $\tau_{\rm ff}$ is the fringe-fit interval in seconds, which should be less than or about equal to the coherence time.
For non-identical antennas 1 and 2, SEFD can be replaced by the geometric mean $\sqrt{{\rm SEFD}_1 \times {\rm SEFD}_2}$.